1. Field of the Invention
The invention pertains to the field of variable cam timing systems. More particularly, the invention pertains to a variable cam timing phaser with an offset spool.
2. Description of Related Art
Internal combustion engines have employed various mechanisms to vary the angle between the camshaft and the crankshaft for improved engine performance or reduced emissions. The majority of these variable camshaft timing (VCT) mechanisms use one or more “vane phasers” on the engine camshaft (or camshafts, in a multiple-camshaft engine). In most cases, the phasers have a housing with one or more vanes, mounted to the end of the camshaft, surrounded by a housing with the vane chambers into which the vanes fit. It is possible to have the vanes mounted to the housing, and the chambers in the housing, as well. The housing's outer circumference forms the sprocket, pulley or gear accepting drive force through a chain, belt or gears, usually from the camshaft, or possibly from another camshaft in a multiple-cam engine.
The spool valve of the variable cam timing phasers may be mounted externally from the phaser or internal to the phaser. The internally mounted spool valve may be center mounted and some of the limitations of center mounting of a spool are having to use a center bolt to mount the spool valve as shown in Butterfield et al.'s U.S. Pat. No. 5,046,460, mounting the spool valve in the camshaft end as in Butterfield et al.'s U.S. Pat. No. 5,002,023, or using a flange on the end of the camshaft to mount the spool valve as in Becker et al.'s U.S. Pat. No. 5,107,804.
An example of an internal center mounted spool in a variable cam timing (VCT) phaser is shown in prior art FIG. 1. The VCT phaser 22 is coupled to a camshaft by numerous bolts 36. The housing 40 of the phaser has an outer circumference or teeth 56 for accepting drive force from a chain 58. The rotor 38 is connected to the camshaft and is coaxially located within the housing 40. The rotor 38 has vanes 42, which separates chambers formed between the housing 40 and the rotor 38 into advance chambers 46 and retard chambers 48. The vanes 42 are capable of rotation to shift the relative angular position of the housing 40 and the rotor 38. Fluid is supplied to the phaser 22 through supply line 55 leading to the spool valve 50. Lines 52, 54, 60, supply fluid between the advance 46 and retard chambers 48 and the center mounted spool valve 50. Check valves 61 are present in line 54. The position of the spool within the spool valve 50 controls the motion, (e.g. to move towards the advance position or the retard position) of the phaser.